Fracture strength characterization for 25 micron and 125 micron thick SOI-MEMS structures

Abstract

MEMS structural devices are often fabricated by bulk-micromachining using a deep reactive ion-etching process (DRIE) for silicon. DRIE creates the high aspect ratio silicon features used as sensors and actuators in MEMS devices. This paper characterizes fracture strength distributions for DRIE etched MEMS test structures with device layer thicknesses of 25 μm and 125 μm using pull-table style strength test structures. The 25 μm thick measurements offered a direct comparison with previous investigations; whereas, the 125 μm thick measurements are unique to this study. The fracture strength distributions measured for 125 μm thick specimens ranged to significantly lower values and had a narrower distribution compared to previous and current 25 μm thick device layer fracture strength distributions. scanning electron microscopy (SEM) and atomic force microscopy (AFM) were used to characterize and correlate surface roughness features with the observed fracture strength measurements. Characterization revealed that although both the 25 μm and 125 μm thick device layer structures predominantly exhibited scalloping sidewall morphology, the 125 μm thick structures transitioned to rougher curtaining sidewall morphology towards the bottom of the device layer. The difference in strength distributions is attributed to this increased roughness present in the curtaining region and on the bottom surfaces of thicker device layers.

An SOI-based post-fabrication process for compliant MEMS devices

“…The SEM measured result shows a 0.5 µm reduction in structure width. Taking into account fabrication error, the adhesive force caused maximum stress of the microgripper without the tether is 1.01 GPa, corresponding to a failure probability of 32% [24]. Thus, the survival probability of the microgripper without the tether is 46% after fabrication, which matches well with the actual value of 44%.…”

“…The microgripper is likely to be broken if the stress exceeds the fracture strength of silicon. The fracture strength of the DRIE silicon structure ranges from 600 MPa to 3.5 GPa and complies with the Weibull distribution [24]. The failure probability with stress around 887 MPa is about 20%.…”

Optimization of the Geometry of a Microelectromechanical System Testing Device for SiO2—Polysilicon Interface Characterization

“…• The failure-governing principal stress component at the interface should be as high as possible, ideally close to 1 GPa, in order to speed up the fatigue tests. • The stress field in the polysilicon layer should not exceed the one at the interface to avoid inducing brittle cracking in the latter region [28][29][30][31][32][33][34][35][36][37][38][39].…”